Jose Fernandez had one of the greatest breaking balls in an era where pure stuff has never been better.
The pitch, which was given the nickname "The Defector", is classified by PITCHf/x as a curveball. It often took a slurve type shape, with a ton of glove side break. Early in his career, the curve sat around 82 mph with about 9 inches of glove side movement, which ranked at the top of the curveball movement leaderboard in his rookie year in 2013. In 2016, the pitch picked up about 2 mph from his rookie season, up to just under 84 mph on average, but traded the increased velocity for about two inches less glove side movement.
The pitch generated incredible outcomes. In the PITCHf/x era, Fernandez's curveball ranks as the #3 most effective curveball among qualified starting pitchers by both slugging percentage against (.220) and batting average against (.140). Generational lefty Clayton Kershaw and 2014 Cy Young winner Corey Kluber are the only pitchers who have generated better outcomes with a curveball.
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It was a huge swing and miss pitch. Fernandez's curve generated a swinging strike rate of 20.5%, way above the MLB average of about 11% on curveballs in the PITCHf/x era and about 12% on curveballs in 2016. His 505 swinging strikes on his curveball ranks #7 in the PITCHf/x era, and he did it in a fraction of the total pitches thrown by the players ahead of him. Fernandez didn't make his MLB debut until 2013--and missed a year in between recovering from Tommy John surgery--while the tracking system has been logging swinging strikes dating back to 2008.
Part of the effectiveness of the pitch was that Fernandez was a master at playing it off his mid to high 90s fastball. They both came from a similar arm slot, and his curve broke so late that it looked like a fastball to hitters out of his hand before diving out of the zone. Hitters often couldn't tell what was coming because of the deception. But the actual pitch itself was filthy.
It wasn't just the deception and the pure filth that made it so good. His command of the pitch was strong. Here's a visual from his first month in the big leagues, where he bends an 80 mph curve that starts well off the plate and perfectly nails the corner for a called third strike.
Fernandez could consistently put his curveball in places that looked like balls before breaking into the zone and in places that looked like strikes before breaking out of the zone.
Like a lot of other baseball fans, I've had the thought cross my mind that if I'm fortunate enough, I will someday tell my kids and grandkids about Jose Fernandez and how great he was. And the first thing I'll talk about is that crazy good fastball/curveball combo he played off each other and the ridiculous late break on his curve.